No indeed, that's too costly. With this approach you can only receive messages from someone after you have added the corresponding public key in your client. This is an extra step to start messaging, but does eliminate spam.

It's probably exactly the same, messages sent on one should be readable on the other

I think it was always planned to update the spec in the future, you might be right that it's not super secure

404 should be fixed!

refreshing the login page gives an error indeed, i need to fix that. You do need a nostr browser extension as well. you can get nos2x or the alby browser extension.

It works for me?

How does the recipient not know the full key to decrypt it? It seems to be working

It's actually not that advanced. Doesn't provide forward secrecy like signal. Can and will be improved upon in the future.